Sealing porous dielectric materials

ABSTRACT

Method and structure for minimizing the downsides associated with microelectronic device processing adjacent porous dielectric materials are disclosed. In particular, chemical protocols are disclosed wherein porous dielectric materials may besealed by attaching coupling agents to the surfaces of pores. The coupling agents may form all or part of caps on reactive groups in the dielectric surface or may crosslink to seal pores in the dielectric.

This is a Divisional Application of Ser. No. 10/735,122 filed Dec. 12,2003, which is presently pending.

BACKGROUND

1. Field of the Invention

This invention relates to dielectric layers with low dielectricconstants, and more particularly to forming barriers across exposedpores in porous low dielectric constant dielectric layers.

2. Background of the Invention

Low dielectric constant (“k”) materials are used as interlayerdielectrics in microelectronic devices, such as semiconductor devices,to reduce the resistance-capacitance (“RC”) delay and improve deviceperformance. As device sizes continue to shrink, the dielectric constantof the material between metal lines must also decrease to maintain theimprovement. Certain low-k materials have been proposed, includingvarious carbon-containing materials such as organic polymers andcarbon-doped oxides. The eventual limit for a dielectric constant isk=1, which is the value for a vacuum.

One of the challenges encountered in microelectronic device processingrelates to the diffusion of wet chemical and processes gases throughdielectric films leading to increased k values (water has a k value ofabout 80). While pores left in dielectric thin films after certainprocess steps may be advantageous from a k-value perspective when dry(dry air or nitrogen having relatively low k values), they may alsofacilitate fast diffusion of unwanted moisture, and may increase surfacereactivity with such moisture due to the increased accessible surfacearea provided by pores, along with possible hydrophilic SiOH formationto propagate such a problem. SiOH available to react near the surface ofdielectric pores may result in an increased k value. Pores may alsocause additional challenges to subsequent process steps due to geometricconsiderations. For example, forming a thin sidewall upon a trench cutinto a highly porous dielectric material presents obviouschallenge—particularly if one of the sides of the trench upon which asidewall is to be formed lies in the middle of a large pore.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited in the figures of the accompanying drawings, in which likereferences indicate similar elements. Features shown in the drawings arenot intended to be drawn to scale, nor are they intended to be shown inprecise positional relationship.

FIGS. 1 a and 1 b are cross sectional side views of a porous dielectriclayer.

FIG. 2 is a flow chart that describes how porous dielectric may besealed.

FIG. 3 a is an illustration of a coupling agent used according to oneembodiment.

FIG. 3 b illustrates the linkage of the coupling agent to the poresurface.

FIG. 3 c illustrates the formation of the barrier across the opening ofthe pore.

FIG. 4 a illustrates the dielectric exposed to a coupling agent and theresults of various reactions that may occur.

FIG. 4 b illustrates exposure of coupling structures to an additionalsealing agent.

FIG. 5 illustrates another example of how a porous dielectric may besealed.

FIG. 6 illustrates yet another example of how a porous dielectric may besealed.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements. The illustrative embodiments described hereinare disclosed in sufficient detail to enable those skilled in the art topractice the invention. The following detailed description is thereforenot to be taken in a limiting sense, and the scope of the invention isdefined only by the appended claims.

FIGS. 1 a and 1 b are cross sectional side views of a porous dielectriclayer. Referring to FIG. 1 a, there is a porous dielectric layer 104above a substrate layer 102. The porous dielectric layer 104 has pores108, some of which are within the dielectric layer 104, and some ofwhich are exposed at the surface. In an embodiment, the pores 108typically have a size ranging from about 20 angstroms to about 100angstroms, although the pores may also be other sizes. There is also atrench 106 formed in the dielectric layer 104. As shown in FIG. 1 a, thepores 108 at the surface of the dielectric layer 104 or at the sidewallsof the trench 108 may have openings that expose the interior of thepores 108 to the surrounding environment. The pores 108 may make iteasier for materials in the environment to diffuse into the dielectricmaterial 104, may increase the difficulty of forming thin films on thedielectric layer 104. The porous dielectric material 104 may havereactive groups, such as SiOH groups, near the surface that may cause anincrease the dielectric constant (“k”) value of the dielectric layer104.

Referring to FIG. 1 b, barriers 110 have been formed over some of theopenings of exposed pores 108 at the surface of the dielectric layer104. These barriers 110 may be considered to “seal” the pore 108 or“seal” the dielectric layer 104 and may prevent problems such as thosedescribed above. A pore 112 illustrated in FIG. 1 b does not have abarrier formed covering it, but a reaction has caused reactive SiOHgroups formerly found near the surface of the dielectric 104 to becapped, which may prevent an increased k value of the dielectric layer104. Such a capping of SiOH or other reactive chemical structures on thesurface of the dielectric 104 may also be considered as a “sealing” ofthe pore 108 or a “sealing” of the dielectric 104.

FIG. 2 is a flow chart 200 that describes various ways that a porousdielectric, such as dielectric 104, may be sealed according to someembodiments of the present invention. This sealing may include cappingreactive chemical structures, such as SiOH groups, of the dielectricmaterial. The sealing may include forming barriers 110 over exposedpores 108. To seal the dielectric 104, the dielectric 104 may be exposed202 to a coupling agent. The coupling agent may then link 204 to thedielectric 104 surface. This linking may result from a reaction betweenthe coupling agent and chemical groups at the dielectric 104 surface toform a coupling structure attached to the dielectric 104 surface.

In a first sealing embodiment, the coupling agent and the dielectric 104may be exposed 206 to a crosslinking agent. This crosslinking agentexposure 206 may cause the coupling agent or coupling structure attachedto the dielectric 104 to crosslink in a reaction. Such crosslinks mayseal 208 the dielectric 104. This seal 208 may result from crosslinkingthat form barriers to prevent external material from penetrating thepores 108, or crosslinked groups capping reactive groups on the surfaceof the dielectric 104.

In a second sealing embodiment, the coupling agent and the dielectric104 may be exposed 210 to a capping agent. This capping agent exposure210 may cause the coupling agent or coupling structure attached to thedielectric 104 to react to form 212 a cap structure to seal thedielectric 104. Note that both a crosslinking agent and a capping agentmay be referred to as a “sealing agent.”

In a third sealing embodiment, the coupling agent may seal 214 thedielectric 104 without additional capping or crosslinking agents. Thecoupling agent may react with the surface of the dielectric 104 to forma cap to seal the dielectric 104. Alternatively, coupling structureslinked 204 to the dielectric 104 surface may crosslink with othercoupling structures linked 204 to the dielectric 104 to form a barrierand seal 214 the dielectric 104. This seal 214 may result fromcrosslinks that form barriers to prevent external material frompenetrating the pores 108, or from crosslinked groups or capped groupsthat cap reactive groups on the surface of the dielectric 104.

In other embodiments, the dielectric 104 may be sealed using multipleones of the first through third embodiments, or may be sealed throughother methods. For example, in a fourth sealing embodiment, thedielectric 104 may be exposed 202 to a coupling agent that may formcoupling structures linked 204 to a pore 108 surface. Some couplingstructures may partially seal 214 the pore 108 by crosslinking or otherprocesses, without being exposed to a crosslinking or other agent. Othercoupling structures may crosslink in response to being exposed 206 to acrosslinking agent, and form 208 a barrier to seal the pore 108, or capreactive groups on the pore 108 surface. Other coupling structures mayalternatively form 212 a cap structure in response to being exposed 210to a capping agent. Thus, in the fourth sealing embodiment, thedielectric 104 may be sealed partially via the first or second sealingembodiments and partially via the third sealing embodiments. Otherembodiments of sealing a dielectric material 104 by capping orcrosslinking may also be used.

In some embodiments, a cleaning step (not shown) may also be performed.In such a cleaning step, products of chemical reactions, excess couplingor crosslinking agents, or other substances may be removed from thevicinity of the dielectric 104. This may be accomplished by a flow ofgas or fluid removing the substances, causing a reaction of thesubstances with another material to result in an acceptable resultmaterial, or by other methods. Other additional steps may also beperformed in some embodiments.

FIGS. 3 a through 3 c illustrate how porous dielectric material 104 maybe sealed according to an example of the first sealing embodimentdescribed above with respect to FIG. 2. FIG. 3 a is an illustration of acoupling agent 300 to which a pore 108 in the dielectric 104 may beexposed 202 in one embodiment. The coupling agent 300 depicted in FIG. 3a is a silane coupling reagent 300 with a thiol (“SH”) termini or “endcap” 302 that may facilitate disulfide bonding to seal an exposed pore108. The silane coupling reagent 300 is depicted with the thiol end cap302 comprising a sulfur atom that may form one half of a disulfide bondat the end of the chemical progression. The thiol end cap 302 may becoupled to a silicon atom 308 by a chain 306 of CH₂ moieties. The lengthof the chain 306 may be selected to facilitate micromotion of the thiolend cap 302 for sulfur-sulfur (“disulfide”) bonding. In other words, thechain of CH₂ moieties may provide a relatively “floppy” construct toallow the thiol end cap 302 freedom to move into electrical contact withanother nearby thiol end cap 302, enabling formation of a disulfidebond. Such a floppy construct may be known as a flexible chain. In someembodiments, the chain 306 may be relatively long. In an embodiment, thechain 306 may have 4 CH₂ moieties, and in some other embodiments, thechain 306 may have from about 3 to about 5 CH₂ moieties.

The depicted embodiment of the silane coupling reagent 300 alsocomprises three surface coupling groups 304 selected to react with SiOHgroups or other groups that might be found on the surface of a silicateglass pore 108. These surface coupling groups 304 need not be the exactOCH₃ (or CH₃O) groups depicted—indeed, they may be referred to as“alkoxy,” “ether,” or “alkoxides” to one skilled in the art. Forexample, anywhere from one to three ethoxy groups would work toreactively link with the surface chemistry at the pore 108 in anembodiment. Other reagent surface coupling groups, including but notlimited to those known as tert-butoxy and isopropoxy, may be substitutedinto the silane coupling reagent 300 as surface coupling groups 304. Inan embodiment, the alkoxy (oxygen) groups support pore surface reaction.

Referring now to FIG. 3 b, the linking 204 of the coupling agent 300 tothe pore 108 surface is illustrated according to one embodiment. In anembodiment, the material 104 may be exposed to nitrogen or helium gascontaining the coupling agent 300 at a concentration below the couplingagent's 300 lower flammability limit and at a temperature just below theflash point of the coupling agent 300. As the silane coupling agent 300is introduced, nearby water 310 may react with the surface couplinggroups 304, OCH₃ in this embodiment, to form methanol (“CH₃OH”) 312. Themethanol 312 may readily evaporate or may be vented away. This resultsin SiOH groups (not shown) that react with other SiOH groups alreadyhanging off the surface of the pore 108 to form “Si—O—Si” linkages, asdepicted in FIG. 3 b. The silicon atom 308 is therefore coupled to thesurface of the pore 108. Thus, the reaction of the coupling agent 300has resulted in a coupling structure linked 204 to the surface of thepore 108.

Additional water produced by such reactions (not shown) may help topropagate more of these reactions by facilitating production of moreSiOH groups, and eventually the pore 108 may be substantially occupiedby variations of the silane coupling reagent 300 bonded to the surfaceof the pore 108. As the pore 108 may be substantially occupied by thecoupling reagent 300 bonded to the surface of the pore 108, this mayhave resulted in the capping of the SiOH groups and a sealed pore.

Pore 108 sizes may generally be on the order of 20-100 angstroms in someporous dielectric materials used to form the dielectric layer 104.Silane coupling reagents 300 such as that depicted may have molecularradii of about 3 angstroms. For a pore 108 size of about 20-100angstroms, therefore, about 6-30 molecules of the silane coupling agent300 may be adequate to link 204 with the pore 108 surface and form abarrier for the pore 108 that may prevent external material frompenetrating into the pore 108.

To form a barrier, the linked coupling agent 300 and pore 108 may beexposed 206 to a crosslinking agent 314. In an embodiment, thecrosslinking agent 314 may be a mild oxidizing agent 314, such asformaldehyde (“H₂CO”). In an embodiment, this exposure 206 may occurwhen the pore 108 is substantially covered with linked 204 couplingagent 300 molecules. The oxidizing agent 314 may be selected to bestrong enough to oxidize the “SH” bonds of the thiol end caps 302without substantially oxidizing other adjacent sensitive materials, suchas copper, in an embodiment. In an embodiment, the pore 108 may beexposed to nitrogen or helium gas containing formaldehyde gas as thecrosslinking agent 314, at a concentration below formaldehyde's lowerflammability limit, and at a temperature just below formaldehyde's flashpoint. In an embodiment, the concentration may be below about 7%formaldehyde, and the temperature may be below about 59 degrees Celsius.

FIG. 3 c illustrates sealing 208 the dielectric 104 pore 108 byformation of a barrier across the opening of the exposed pore 108according to one embodiment. The crosslinking agent 314 may oxidize the“SH” bonds of the thiol end caps 302 of the linked 204 coupling agent300 molecules. This enables the sulfur atoms in the end caps 302 tofulfill their bonding capabilities and form a disulfide bond 316 bypairing with another sulfur atom missing an electron. This bond mayresult in a “bridge” structure of multiple coupled barrier molecules,which may each include a silicon atom 308, a flexible chain 306, and asulfur atom. The resultant “bridge” structure across the pore 108 maysubstantially fill the pore opening, creating a barrier 110 to the pore108. The bridge structure may be crosslinked to have enough density toact as a physical barrier to other chemicals getting through as a resultof subsequent process treatments of the dielectric layer 104. Thus, theexposed pore 108 has been sealed. In some embodiments, the crosslinkingmay not result in a bridge that substantially fills the opening of thepore 108. The dielectric 104 may still be sealed, since the crosslinkedstructures may effectively cap reactive groups, such as SiOH groups, atthe surface of the dielectric.

Optionally in some embodiments, the structure may be made morehydrophobic by introducing, for example, hexamethyldisilazane (“HMDS”)or other chemicals. Such a process would further prevent pore reactivityor formation of further SiOH. HMDS forms strong surface bonds, whichcreates a locally hydrophobic environment.

FIGS. 4 a and 4 b illustrate how the dielectric 104 may be sealedaccording to another example of the first sealing embodiment describedabove with respect to FIG. 2, according to an example of the secondsealing embodiment described above with respect to FIG. 2, and accordingto an example of the third sealing embodiment described above withrespect to FIG. 2.

FIG. 4 a illustrates the dielectric 104 exposed 202 to a coupling agentand the results of various reactions that may occur. In the embodimentillustrated in FIG. 4 a, the coupling agent 400 is phosgene, which maybe present in a vapor form. Note that care should be taken when usingphosgene. Even brief exposures to phosgene at a concentration above 50ppm may be fatal to humans. The dielectric 104 may have reactive SiOHgroups 404 at the surface. In an embodiment, phosgene may be introducedin a vapor phase at a temperature in a range from about zero degreesCelsius to about 50 degrees Celsius at a concentration of about 5 ppm toabout 1%. The phosgene coupling agent 400 may react with the SiOH groups404. This reaction may result in HCl 402, and the phosgene 400 linking204 to the surface of the dielectric 104, which may be in a pore 108.The linked 204 phosgene 400 may result in a pendant phosgene functionalgroup 408, which may be considered a coupling structure, linked 204 tothe pore 108 or dielectric 104 surface. Nearby coupling structures mayreact to form disilyl carbonate 406. The disilyl carbonate 406 may seal214 the pore 108, and may comprise an example of the third sealingembodiment described above with respect to FIG. 2 a. The disilylcarbonate 406 may seal the dielectric 104 by forming a pore 108 barrier110 to prevent external material from penetrating the pores 108, and/orby capping reactive groups on the surface of the dielectric 104.

FIG. 4 b illustrates exposure of coupling structures, such as thependant phosgene functional groups 408, to an additional sealing agent410. The agent 410 may be either or both of a crosslinking or cappingagent, so that the coupling agent structures may be exposed 206 to acrosslinking agent according to the first sealing embodiment describedabove with respect to FIG. 2 and/or exposed 210 to a capping agentaccording to an example of the second sealing embodiment described abovewith respect to FIG. 2. Both a crosslinking agent and a capping agentmay be considered “sealing agents” 410.

In an embodiment, the agent 410 may be a crosslinking agent such as amultifunctional alcohol. Examples of such a multifunctional alcohol mayinclude ethylene glycol, propylene glycol, glycerol, erythritol, andpentaerythritol. The alcohol may be in a vapor form. These crosslinkingagents may react to attach to the coupling structures 408, then may formcrosslinks 414 to connect two or more coupling structures 408. In anembodiment where one desires to seal exposed surfaces of the material104, the sealing agent 410 may be introduced as a liquid or in a solventat a temperature in a range from about zero degrees Celsius to about 100degrees Celsius at a concentration of about 0.1% to about 100% (liquidstate). In an embodiment where one desires the sealant to penetratefurther into the material 104, the sealing agent 410 may be introducedas a solution in a supercritical fluid at a concentration of about 0.1%to about 100% or in a vapor phase at a temperature in a range from about100 degrees Celsius to about 300 degrees Celsius at a concentration ofabout 5 ppm to about 5%. Such crosslinking may effectively seal 208 thedielectric 104 by removing reactive SiOH groups at the surface of thedielectric 104 and/or forming a barrier to prevent external materialfrom penetrating the pore 108.

In another embodiment, the agent 410 may be a capping agent such as amonofunctional alcohol. Such a monofunctional alcohol may be methanol.The alcohol may be in a vapor form. In an embodiment, a methanol cappingagent may be introduced as a liquid or in a solvent at a temperature ina range from about zero degrees Celsius to about 50 degrees Celsius at aconcentration of about 0.1% to about 100% (liquid state). In anembodiment where one desires the sealant to penetrate further into thematerial 104, the methanol capping agent may be introduced as a solutionin a supercritical fluid at a concentration of about 0.1% to about 100%or in a vapor phase at a temperature in a range from about 50 degreesCelsius to about 100 degrees Celsius at a concentration of about 5 ppmto about 5%. The capping agent 410 may react to attach to the couplingstructures 408 and form 212 a cap 412 on them. For example, whenmethanol is used as a capping agent, the methanol may react to acoupling structure 408 to form mehtyl silyl carbonate, which may sealthe pore 108 or dielectric 104.

FIG. 5 illustrates how dielectric 104 or pores 108 may be sealedaccording to an example of the third sealing embodiment described abovewith respect to FIG. 2. The dielectric 104 may be exposed 202 to acoupling agent 500. The coupling agent 500 may be an acyl dichloride,such as succinyl chloride, and may include one or more R groups 502 thatmay react to form crosslinks. The R group 502 may be different fordifferent coupling agents 500. For example, in an embodiment where thecoupling agent 500 is succinyl chloride, the R group 502 may be CH₂CH₂.In an embodiment where succinyl chloride acts as a coupling agent 500,the succinyl chloride may be in a solution of a supercritical fluid at atemperature in a range from about zero degrees Celsius to about 100degrees Celsius. The coupling agent 500 may both link 204 to thedielectric surface 104 and crosslink to seal 214 the pore. Thedielectric 104 may have reactive SiOH groups 504 at the surface. Thecoupling agent 500 may react with the SiOH groups 504 to form couplingstructures 506 linked to the dielectric 104 surface, and crosslinkinggroups 502 of the structures 506 may react to form a crosslink 508. Suchcrosslinked structures 506 may seal 214 the dielectric 104. Suchcrosslinking may effectively seal 214 the dielectric 104 by removingreactive groups at the surface of the dielectric 104 and/or forming abarrier to prevent external material from penetrating the pore 108.

FIG. 6 illustrates how dielectric 104 or pores 108 may be sealedaccording to another example of the third sealing embodiment describedabove with respect to FIG. 2. The dielectric 104 may be exposed 202 to acoupling agent 600. The coupling agent 600 may be an acyl chloride, suchas acetyl chloride, and may include a capping group 602. The couplingagent 600 may be introduced under conditions similar to those describedabove with respect to the sealing agent 410. The coupling agent 600 maylink 204 the dielectric surface 104. The dielectric 104 may havereactive SiOH groups 604 at the surface. The coupling agent 600 mayreact with the SiOH groups 604 to form structures 606 linked to thedielectric 104 surface. The capping groups 602 may comprise caps 608 toseal 214 the dielectric 104.

In the embodiments described above, larger coupling agents with morecrosslinking groups may be used to increase the crosslinking density andform a pore 108 barrier 110 to prevent external material frompenetrating the pores 108. Smaller coupling agents with fewer functionalcrosslinking groups may be used to cap reactive groups on the surface ofthe dielectric 104. Further, the depth of the sealing into thedielectric 104 may be controlled by the time which the dielectric 104may be exposed to coupling agents. A smaller exposure time may result insealing the surface of dielectric 104 and pores 108 near the surface,while a larger exposure time may result in sealing the surface of pores108 deeper within the dielectric material.

Thus, a novel pore sealing solution is disclosed. Although the inventionis described herein with reference to specific embodiments, manymodifications therein will readily occur to those of ordinary skill inthe art. Accordingly, all such variations and modifications are includedwithin the intended scope of the invention as defined by the followingclaims.

1. A device comprising: a substrate layer, a porous dielectric layeradjacent the substrate layer with an exposed pore having an opening,wherein the exposed pore is disposed on at least one of a surface of theporous dielectric layer and a sidewall of a trench disposed within theporous dielectric layer; and a barrier across the opening of the exposedpore, wherein the barrier comprises a first barrier molecule with asilicon atom coupled to a surface of one side of the exposed pore, asulfur atom, and a flexible chain between the silicon atom and thesulfur atom, wherein the flexible chain comprises a portion of a bridgestructure that is capable of sealing the surface of the exposed pore,and a second barrier molecule with a silicon atom coupled to a surfaceof an opposite side of the exposed pore, wherein the first and secondbarrier molecules are connected to each other across the surface of theexposed pore.
 2. The device of claim 1 wherein the flexible chaincomprises a substantially long chain of CH₂ groups.
 3. The device ofclaim 2 wherein the substantially long chain of CH₂ groups comprises atleast four CH₂ groups.
 4. The device of claim 1 wherein the exposed poreis in a range from about 20 angstroms to about 100 angstroms, and thebarrier comprises about 6 to about 30 crosslinked barrier molecules.